The research and development of new MEMS technologies and electronic materials for the detection of toxic and explosive vapors with high sensitivity and selectivity is of utmost importance for many commercial, environmental, security applications and for US military missions.
Detection techniques include a variety of physical and chemical methods related to changing the output signal under exposure to target analytes. Physical methods include: nuclear quadrupole resonance, ion mass spectroscopy, gas chromatography, X-ray diffraction, electron capture detection, and laser photofragmentation. These techniques are selective enough, but can be expensive, bulky and cannot be employed for fast, real-time, and remote analyte detection. Also, most of the chemical sensors have been studied, developed, and fabricated in the macro format using traditional techniques for the deposition of sensory polymers (spin-casting, coating, spraying) onto relatively large area substrates followed by coupling to a separate detection/acquisition system (Walt et al., Chem. Rev. 100: 2595, 2000; Grate, Chem. Rev. 100: 2627, 2000). Such devices could be employed for pattern recognition of vapor mixtures. However, vapor concentration should be high enough to prevent a false response and to correctly identify the explosive chemical signature. Since many toxic and explosive vapors (for example, TNT, RDX, PETN) are related to low pressure vapors, the critical issue becomes an enhancement of sensor sensitivity and selectivity to provide a fast, real-time response with a minimum false alarm.
The series of articles and patents by Swager et al. (J. Am. Chem. Soc. 120: 11864, 1998; U.S. Pat. Nos. 7,208,122 and 7,393,503) propose a new concept, namely the “molecular wire” approach, related to emissive optochemical sensors for the detection of explosive vapors. The major issue here is the amplification mechanism based on an energy migration effect allowing very high device sensitivity, which is of utmost importance for the detection of explosives with a low pressure of saturated vapors. U.S. Pat. No. 6,686,206 and an article (J. Phys. Chem. B105:8468, 2001) by Levitsky et al. also describes the optochemical sensors involving amplification mechanism of luminescence, however it is based on the direct Forster energy transfer. Despite possessing high sensitivity, the above emissive sensors suffer low selectivity as quenching or enhancing of the emission demonstrates similar behavior for different parts of the luminescent spectrum.
U.S. Pat. No. 7,419,636 (Aker et al.) describes an instrument for the detection of explosives using fluorescence quenching of amplifying polymers as a transduction mechanism. U.S. Pat. Nos. 7,208,122 and 7,393,503 (Swager et al.) describe the method to synthesize fluorescence amplifying polymers for nitroaromatic explosive detection. Note, that other amplifying polymers (which are not subjected to U.S. Pat. Nos. 7,208,122 and 7,393,503), small emissive molecules and oligomers can also be sensitive to nitroexplosive (see L. Zang, et al. J. Am. Chem. Soc. 129: 6978, 2007; A. Su, et al. Synth. Met. 144: 297, 2004; W. C. Trogler, et al. Angew. Chem. Int. Ed. 40: 2104, 2001; G. Li, et al. Colloid. Polym. Sci. 285: 721, 2007). All of these species (emissive sensory organics) can be infiltrated inside one-dimensional porous photonic crystal with microcavity (MC) forming a novel nanocomposite emissive material with advanced sensory characteristics to explosive vapors and particulates. The method of explosive and other low pressure vapors detection using MC based emissive nanomaterials has been described in the U.S. patent application Ser. No. 12/051,233 (Levitsky). This method has serious advantages over traditional fluorescence quenching because of the additional sensory parameter: MC spectral shift upon vapor exposure. As a result, an enhanced selectivity can be achieved. Also, nanoporous structure of photonic crystal results in much higher surface area (ranging from 200 to 800 m2/cm3), which provides numerous sites between the sensory material and the analyte vapors. This increases sensitivity and reduces response time. The method described in the U.S. patent application Ser. No. 12/051,233 was not implemented in the design of a real device capable to detect low concentrated vapors and particulates in the real-time mode and also vapors with moderate and high vapor pressure. Also, in this application nothing has yet been disclosed about real-time sampling system and no details were presented about the preparation of the MC based emissive sensory material.
It would therefore be desirable to have a thoughtful description of the sensory device (including sampling system) for optochemical detection vapors and particulates using MC based emissive composite materials.